Metal filters, such as porous stainless steel filters, have found use in a variety of processes where high pressure drops are encountered and in applications where fine filtration capability must be combined with mechanical strength, resistance to high temperatures and/or resistance to chemical attack.
An example of an application for porous stainless steel filters is the filtration of fine catalysts used in fluid catalytic cracking processes in the petroleum industry. Temperatures as high as several hundred degrees Fahrenheit may be encountered by the filters used in such processes. The catalysts being filtered include various oxides such as silica, alumina, magnesia, zirconia and the like as well as molecular sieves. When these materials plug the pores of a fine-pored filter, they are very difficult to remove by conventional means such as backflushing.
Porous stainless steel filters have also found wide acceptance in the manufacture of high fidelity magnetic recording tapes which use various magnetic materials, such as iron oxide or chromium oxide in conjunction with various binder resins. In this application, the filters ultimately become plugged with a mixture of (1) organic material, i.e., the binder resins present in the compositions being filtered, and (2) metal oxide(s). Again, removal of these materials by conventional means is very difficult.
Still another use of such filters is in the filtration of the resin streams used in the manufacture of polymers as, for example, polyesters. Typically, in such applications a high temperature feed stream of a monomer containing a catalyst must be filtered, ultimately leading to the plugging of the filter with a crosslinked gelled material which renders cleaning of the filter very difficult.
Since porous metal filters such as those used in the processes described above are quite expensive, e.g., on the order of 100 times as expensive as comparable sized conventional filters utilizing cellulosic fibers or the like, there is tremendous incentive to clean these filters and return them to use as opposed to simply replacing them at considerable expense. In spite of this incentive, prior to this invention there has, for the most part, not been a really satisfactory method for cleaning such filters and returning them to operation. This invention, then is directed to a process for cleaning such filters and, indeed, a process which, when operated in the optimum manner, can clean them so effectively that, for the most part, they are comparable to new filters of the same design.